Bug 1356976 - Don't make mach artifact toolchain return an error code when there is nothing to download. r=gps

There can be cases where there is simply nothing to download, especially
with the --for-job argument. So we just stop erroring out when nothing
was downloaded.

However, if the user explicitly requested a particular file(s) via the
command line and there is nothing to download, we still emit an error
code.

python/mozbuild/mozbuild/mach_commands.py

@@ -1778,7 +1778,8 @@ class PackageFrontend(MachCommandBase):
 
         if not downloaded:
             self.log(logging.ERROR, 'artifact', {}, 'Nothing to download')
-            return 1
+            if files:
+                return 1
 
         return 0